05000362/LER-2008-002
| San Onofre Nuclear Generating Station Unit 3 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 3622008002R00 - NRC Website | |
Event Date:
December 19, 2008 Reactor Vendor: Combustion Engineering Mode: Mode 1 - Power Power 55 Percent
Background:
The Auxiliary Feedwater (AFW) [BA] System automatically supplies feedwater to the steam generators to remove decay heat from the Reactor Coolant System [AB] upon the loss of normal feedwater supply [SJ]. The AFW System consists of two motor driven AFW pumps and one steam turbine driven pump configured into three trains. Each pump can provide, through a common header, 100 percent of the required capacity to the steam generators assumed in the Final Safety Analysis Report (FSAR). The steam turbine driven AFW pump receives steam through a 1E 125 VDC powered control valve (governor) actuated to the position appropriate for motive force to deliver feedwater to the steam generators. To prevent pump overspeed, the steam driven AFW pump is equipped with a mechanical device that actuates due to centrifugal force and trips a linkage to the turbine-stop valve that closes the steam supply to the pump.
Technical Specification (TS) 3.7.5 requires, in part, three AFW trains to be Operable in Modes 1, 2 and 3. TS 3.7.5 requires performance of Surveillance Requirement (SR) 3.7.5.2 in accordance with the Inservice Testing Program to verify Operability of the turbine driven AFW pump. This SR is performed in Mode 3 after reaching 800 psig in the steam generators. TS 3.7.5 Condition B, "One AFW train inoperable", requires restoration of the AFW train to Operable within 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />.
Description of Event:
On December 9, 2008, SCE performed surveillance testing for 3HV4716, the steam inlet valve for the turbine driven AFW pump 3P140. During this work a contract technician removed and replaced a fuse in the control circuitry for 3P140. On December 12, 2008, the circuit was successfully tested in accordance with TS SR 3.7.5.2.
On December 19, 2008, Unit 3 was in Mode 1 at approximately 55 percent power, returning to full power from a planned outage. At about 1651 PST, the Unit 3 Control Room Reactor Operators [Licensed, Utility] received an alarm indicating Turbine Driven Auxiliary Feedwater pump governor out-of-service. In accordance with TS 3.7.5 Condition B, Unit 3 entered a 72 hour8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> Action to restore the inoperable steam driven AFW pump.
SCE personnel took voltage measurements of the turbine driven AFW pump control circuit and found an open circuit across a fuse protecting the 125 VDC supply to the control valve (governor). SCE determined the clip retaining the fuse was sprung, preventing the secure fit necessary for continuity.
The clip was repaired and the AFW pump declared operable at 0330 PST on December 20, 2008 and the TS Action was exited.
Although the circuit was successfully tested in accordance with TS SR 3.7.5.2 on December 12, 2008 (within the 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> allowed by TS SR 3.7.5.2), SCE concluded that the loose clip may have prevented the circuit from performing its required function during a seismic event. SCE's investigation determined that the fuse clip was damaged during work performed on December 9, 2008 (See Cause of the Event below).
TS 3.7.5 required the AFW pump to be operable beginning December 9, 2008 at about 2214 PST, when Unit 3 first entered Mode 3 following a planned refueling outage. Consequently, the AFW pump was in service for about 10 days and 5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> while it may not have been seismically qualified.
Because the allowed outage time for the AFW pump in TS 3.7.5 is 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />, SCE is reporting this event in accordance with 10CFR50.73(a)(2)(i)(B) for a condition prohibited by TS.
SCE had also replaced the turbine stop valve as planned maintenance in the refueling outage.
During this effort, the valve spindle was inadvertently bent but the damage was not immediately apparent. The damage was subsequently discovered during post maintenance testing, which is performed with the plant in Mode 3 and after reaching 800 psig in the Steam Generators (December 10, 2008 at about 0304 PST). When the test was performed, the valve would not close and the damage was subsequently discovered. During the period of TS applicability, the valve was able to perform its safety function (to open on demand). Consequently, this condition did not cause the valve to be considered inoperable and did not result in a TS violation. Subsequently, the valve was reworked and successfully tested (open and close) on December 12, 2008 at about 2355 PST.
Cause of the Event:
SCE's cause evaluation determined the damage to the fuse clip occurred on December 9, 2008 by the technicians performing the planned maintenance. To support work on the system, the technicians involved reported that they pulled the top of the fuse out and pivoting the fuse away, leaving the bottom fuse clip engaged. SCE concluded that this method spread the bottom clip too wide and resulted in the fuse being too loose to ensure continuity during a seismic event.
SCE interviewed a cross section of craft and operators that handle fuses and found that in each case, the technique to pull fuses would completely remove the fuse rather than "hinging" the fuse with one end remaining in the clip. SCE also determined that the worker involved was a contract technician [non-utility, non-licensed]. SCE concluded that the inappropriate fuse pulling technique was a single non-recurring personnel error and limited to this case.
Corrective Actions:
SCE has completed the following corrective actions:
1. SCE replaced the fuse block on the control circuit for the Unit 3 steam turbine driven AFW governor valve.
2. SCE removed and replaced the similar fuse block on Unit 2 (for evaluation).
Planned Corrective actions:
3. SCE will inspect similar design fuse holders in the Diesel Generator and Auxiliary Feedwater systems at both Units 2 and 3. The results of these inspections would then be used to determine if additional inspections are required.
4. SCE will modify applicable plant procedures and installed equipment signage to direct the use of installed disconnect switches instead of pulling fuses, as appropriate.
5.S SCE will revise applicable initial and requalification training to address expectations for removal, installation and verification of proper installation of fuses.
SCE is continuing to evaluate this event. Additional Corrective Actions may be implemented when identified.
Safety Significance:
Although the bent spindle was not reportable, SCE considered both the bent spindle and the loose fuse when evaluating the significance of the event. SCE's Probabilistic Risk Assessment (PRA) included the following factors: (1) the duration of conditions, (2) the likelihood of a seismic event occurring during this period, (3) actual plant conditions, and (4) the availability of redundant plant equipment. The calculated incremental core damage probability (ICDP) and incremental large early release probability (ILERP) are estimated to be 5E-7 and 4E-8, respectively. SCE concluded the risk significance of this event was very small.
Additional Information:
In the past three years, SCE has reported the following similar events:
by a loose electrical connection.
loose electrical connection.
a loose bolted connection between the battery and its associated breaker.